1. Field of the Invention
The present invention relates to a measuring apparatus for sample analysis using an evanescent wave generated when a light beam is totally reflected.
2. Description of the Related Art
In a metal, free electrons oscillate as a group and a compressional wave called a plasma wave is generated. The quantized compressional waves generated on the surface of a metal are known as surface plasmons.
Various surface plasomon sensors have been proposed for quantitative analysis of a substance in a sample by applying the phenomenon that the surface plasmons are excited by a light wave. Among these sensors, the one that uses a system called Kretschmann geometry is particularly well-known as described, for example, in Japanese Unexamined Patent Publication No.6 (1994)-167443.
Basically, the surface plasmon sensor that uses the system described above comprises, for example, a dielectric block shaped like a prism; a metal film formed on one of the surfaces of the dielectric block and brought into contact with a sample; a light source for generating a light beam; an optical system for entering the light beam into the dielectric block at various angles to satisfy the conditions of total reflection at the interface between the dielectric block and the metal film, and to cause attenuated total reflection by surface plasmon resonance; and a light detecting means for detecting the state of surface plasmon resonance or attenuated total reflection by measuring the intensity of the light beam totally reflected at the interface.
In order to obtain a light beam having the various incident angles described above, a comparatively narrow light beam may be entered into the interface by changing its incident angle, or a comparatively wide light beam may be entered thereto as a converging or diverging light beam to include light components that are incident on the interface at various angles. In the first case, the reflected light beam that changes its reflection angle in accordance with its incident angle may be detected by a small light detector that moves in synchronization with the change in the reflection angle, or by an area sensor that extends along the changing direction of the reflection angle. In the latter case, the reflected light beam may be detected by an area sensor that extends along the direction where all of the light components of the light beam reflected at various angles may be captured.
When a light beam enters the metal film of a surface plasom sensor configured in the aforementioned manner at a certain incident angle θsp which is greater than the total reflection angle, an evanescent wave having a distributed electric field in the sample in contact with the metal film is generated, and thereby surface plasoms are excited at the interface between the metal film and the sample. When the wave-number matching is achieved, in which the wave-number vector of the evanescent light matches the wave-number vector of the surface plasmons, the evanescent light and the surface plasmons go into the sate of resonance, and the intensity of the light totally reflected at the interface between the dielectric block and the metal film drops sharply, because the light energy is transferred to the surface plasmons. This drop in the intensity of light is generally detected as a dark line by the light detecting means described above.
The resonance described above occurs only when a p-polarized light beam enters the metal film. Accordingly, arrangements need to be made in advance so that the light beam enters the metal film in p-polarized mode, or only p-polarized light components are detected by the light detecting means.
When the wave-number of the surface plasmons is determined from the incident angle θsp that causes attenuated total reflection (ATR), the dielectric constant of the sample may be obtained. More specifically, the following relationship may be derived, assuming that Ksp as the wave-number of surface plasmons, ω as the angular frequency of the surface plasmons, c as speed of light in vacuum, εm as the dielectric constant of the metal, and εs as the dielectric constant of the sample.             K      SP        ⁡          (      ω      )        =            ω      c        ⁢                                                      ɛ              m                        ⁡                          (              ω              )                                ⁢                      ɛ            s                                                              ɛ              m                        ⁡                          (              ω              )                                +                      ɛ            s                              
When the dielectric constant εs is determined, the density of a particular substance in the sample may be obtained based on a predefined calibration curve, and the like. That is, by determining θsp that causes the drop in the reflected intensity of light described above, the dielectric constant of the sample or characteristics related to the refractive index of the sample may be obtained.
The leakage mode sensor described, for example, on pages 21 to 23, and 26 to 27 of “BUNKOH KENKYU”, Vol. 47, No.1 (1998) is also known as a similar sensor that uses attenuated total reflection (ATR). Basically, the leakage mode sensor comprises, for example, a dielectric block shaped like a prism; a cladding layer formed on one of the surfaces of the dielectric block; an optical guiding layer formed on the cladding layer and brought into contact with the sample; a light source for generating a light beam; an optical system for entering the light beam into the dielectric block at various angles to satisfy the conditions of total reflection at the interface between the dielectric block and the cladding layer, and to cause attenuated total reflection by the excitation of guided mode in the optical guiding layer; and a light detecting means for detecting the state of excitation of guided mode or attenuated total reflection by measuring the intensity of the light beam totally reflected at the interface.
When a light beam is incident on the cladding layer through the dielectric block of a leakage mode sensor configured in the aforementioned manner at a certain incident angle which is equal to or greater than the total reflection angle, certain light components of the light beam having particular wave-numbers and incident angles pass through the cladding layer, and propagate along the optical guiding layer in guided mode. When the guided mode is excited in this manner, attenuated total reflection occurs, in which the intensity of the light totally reflected at the interface described above drops sharply, because most of the light components of the light beam are contained in the optical guiding layer. The wave-number of the guided light is dependent on the refractive index of the sample placed on the optical guiding layer, so that the refractive index of the sample and other characteristics related thereto may be analyzed by determining the particular incident angle that causes the attenuated total reflection described above.
Various types of measuring apparatuses are available that utilize attenuated total reflection of a surface plasmon sensor, leakage mode sensor, or the like, such as an apparatus that enters a light beam containing a plurality of light components of different wavelength into the interface, and detects the level of attenuated total reflection for each wavelength, or an apparatus that splits up a portion of the light beam to be entered into the interface before entering and mixes up the split-up light beam with the light beam reflected at the interface to interfere with each other, and measures the state of the interference, as well as an apparatus that measures a particular incident angle that causes attenuated total reflection described above, in the process of analyzing characteristics of a subject under test by entering light to the interface with an angle that satisfies the conditions of total reflection, and measuring changes in the state of the light totally reflected at the interface due to the evanescent wave generated by the light entered into the interface.
In the conventional surface plasmon sensor, or leakage mode sensor described above, a sample is sometimes replaced together with the dielectric block in order to efficiently conduct measurement for a plurality of samples, when the sample (same measuring unit) needs to be measured a plurality of times at intervals to analyze changes in the state of the sample over time. In this case, when the sample is reset on the measuring apparatus for measurement after replacement, a difference (in inclination) may arise between the initial baseline (interface described above) and the latter baseline. If this difference in inclination between the two baselines is a difference in the vertical inclination that changes the incident angle of the light beam entering at various incident angles, then the reflection angle of the reflected light beam is also deviated, thereby the accuracy of the measurement is lost.
Even when a sample replacement does not take place, changes in the inclination of the baseline may occur subtly by vibrations, or the like, when a plurality of measuring units is moved or rotated on a support or rotating platform. In such a case, changes in the inclination of the baseline developed during a plurality of measurements cause measurement errors.